M R. WALLACE'S last letter seems to call for a word of explanation from me. I did not refer to the up stroke of the bird's wing because this was not the point in dispute. But in reply to Mr. Wallace's latest strictureâ that I appear â to ignore the great downward reaction, added to gravitation, during every up strokeâ â I would say (1) that the downward reaction is not great, (a) because, as Mr. Wallace has himself observed, of the valvular action of the feathers; (b) because of the convex form of the upper surface of the wing; and (c) in some cases, because the wing is less expanded in the up stroke. (2) As to the effect of gravitation, this was already allowed for in determining the resultant motion consequent on the down stroke, and must not be reckoned twice. Just as with an arrow shot from a bow, so with the bird; the motion consequent on the down stroke lasts long enough for the wings to be raised before it is spent. Mr. Wallace is certainly right in saying that the down stroke should counteract the downward reaction of the up stroke, but this downward reaction being slight cannot require â a highly-inclined upward motion,â and what is more, it cannot require that the under surface of the wing should be directed forwards as Dr. Pettigrew asserts.